A. The identity of the informant is probably confidential.
B. The fact that the claims were found to be "unfounded" does not, by itself, get you the ability to sue anyone for making a "false" accusation, just as a "not guilty" verdict in a criminal case doesn't mean that a jury found a defendant factually innocent. It simply means that the investigator did not find enough EVIDENCE of abuse, neglect, etc., to support taking any further steps.
Even if you somehow find out who the informant is, you'll need to prove that they not only made factually false allegations, but they made them knowing that they were factually false.
...and of course, if you hire a lawyer, AND go to trial, AND prove all this, AND get a jury verdict against the informant, you'll have to collect it, which may be difficult unless you have some reason to think the accusation was made by a wealthy person. There's no insurance coverage for this.